I usually use 2-4 chucks of apple/cherry or hickory. Never had an issue with over smoking. 2 birds usually takes 2ish hours. Sometimes more sometimes less. I would suggest a 1/2 full chimney of unlit and then almost a full chimney of lit. This will get your temp up to the 325-350 range where you want to be for just chicken. As with anything, you have to experiment and see what works best for you. Oh and i have fit 3 birds on the top rack with no issue. If you are gonna cook something on the lower, I would either put the chickens in an aluminum foil pan or on the bottom rack. Hope this helps.
Weber Genesis Silver B, Twin 18.5" WSMs, Weber Performer, Green Weber Smokey Joe Gold